It's a fair point. My answer is that laws should be reasonably crafted to prevent or discourage a specific societal ill. When people talk about gun-control, they don't really give a shit about guns. What they really mean is "murder control". Which is fine.
(This is a bad example, but...) Imagine a community is having a rash of smash and grab robberies. In an effort to stop them they outlaw hammers. I just don't believe the new law will work. Robbers will find other tools to smash with, or they'll risk being caught with a hammer since they were willing to risk the much more severe penalty associated with robbery.
Difference is hammers are not Constitutionally protected, and when talking about mass murder you know the perp generally doesn't care about the severity of the penalty. He's gamed it out, and to him the cost (potentially his life) is worth it.